Carbon nanotubes, graphene sheets such as single graphene sheets (SGS), and arrays of these materials (hereafter referred to collectively as graphene-like carbon or GLC) have enormous opportunities for novel electric, mechanical and chemical properties [See references 1-5]. Numerous breakthroughs have been demonstrated successfully and have led to practical fabrication of carbon nanotube electronics devices, such as transistors, interconnects, spintronics, and sensors [See references 6-8].
GLCs are also promising candidate materials for lowest level on-chip interconnect conducting material of future integrated circuits, because they address such issues as signal propagation delay and process integration density and scaling, which are currently limited by Cu and Al metal wires.
However, a fundamental problem with these systems is the nature of the attachment to supports and conducting material. Major issues are mechanical stability, integrity of the electrical contact, and contact resistance between the conducting material that constitutes the support or the electrode (ES) and the GLC, hereafter denoted the ES-GLC interface.
The difficulty in achieving the desired mechanical and electrical properties arises from the nature of the bonding in GLC, which is characterized by sp2 or planar carbons with strong delocalization or resonance in the out of plane pi orbitals. As a result the atoms in these planes interact only weakly with a support or electrode onto which they are attached. This leads to low binding energies (sometimes called van der Waals or Noncovalent bonding) and consequently weak mechanical strength. Concomitantly there is little delocalization of the active electrons the GLC with the ES, leading to a high contact resistance (low current for a given voltage across the interface) and capacitance (due to accumulation of charge as the voltage is ramped with time).
Quantum mechanics (QM) methods (Green's function [see references 16-21] with density functional theory, DFT) have been used to evaluate these mechanical and electrical properties from first principles. It has been shown that the most popular choices for electrodes (Cu, Au, and Pt) in the current settings lead to very poor mechanical and electrical properties. Of the more noble metals (less susceptible to oxidation), Pd has been shown to be the best in the current settings. It has also been shown that electropositive metals, such as Ti (the same would be true for Sc—Ni, Y—Ru, La—Os, Ac-Lw), make good mechanical and electrical contacts in the current settings but may lead to practical problems, since in the current settings they react with the carbon from the GLC and shows a diminished mechanical and electrical contact in presence of oxygen.
The above problems are also present in fuel cell technology. With reference to fuel cell electrodes, transport of protons, electrons, and molecules are all determining factors in high performance fuel cell electrodes [see reference 24] One strategy for enhancing mass and electron transport is to use a nanostructured fuel cell electrode consisting of carbon nanotubes (CNT) loaded with Pt nanoparticles (Pt NPs) [see references 25-28]. The same problems identified above apply to fuel cells manufactured according to this strategy and used the settings currently known.
A further issue relates to the ES material used. Cu is currently the leading on-chip interconnect for integrated circuits, with fabrication processes established in the 1990's that have been well studied and improved since then. [see references 46-49] Advantages of Cu that have led to its extensive use in electronics is that it has the second highest electrical conductivity of pure metals and is abundant and inexpensive. Thus, it would be most desirable to connect Cu electrodes directly to the GLCs. However, it has been previously shown that the Cu-CNT interface in the current settings leads to extremely high contact resistance (11.7 MΩ), 672 times worse than Ti (17.4 kΩ) and 74 times worse than Pd-CNT interface (159 kΩ). [see reference 50] In addition, in the current settings Cu is mechanically weak. [see reference 50] As a result, metal contacts to CNT electrodes have preferred Pd, with little use of Cu.